Since the demonstration of low working voltages by Tang et al. (compare C. W. Tang et al.: Appl. Phys. Lett. 51 (12), 913 (1987)), organic light-emitting diodes have become promising candidates for the realisation of novel illuminating or display elements. They comprise a series of thin layers of organic materials which are preferably vapour-deposited in a vacuum or spin-coated in their polymer form. Following electric contacting by means of metal layers, they form a variety of electronic or optoelectric structural elements such as diodes, light-emitting diodes, photodiodes and transistors. With their respective properties, they provide for competition for the established structural elements on the basis of inorganic layers.
In the case of the organic light-emitting diodes, and by means of the injection of charge carriers, namely electrons from the one side and holes from the other, from the contacts into the adjoining organic layers as a result of an externally applied voltage, the following formation of excitons (electron-hole-pairs) in an active zone and the radiating recombination of these excitons, light is generated and emitted from the light-emitting diode.
The advantage of such structural elements on an organic basis compared with the conventional structural elements on an inorganic basis, for example semiconductors such as silicon, gallium arsenide, is that it is possible to manufacture very large-surface elements, meaning large display elements (monitors, screens). The organic basic materials are relatively inexpensive compared to inorganic materials. Moreover, these materials can be deposited onto flexible substrates because of their low process temperature compared with inorganic materials. This fact opens the way to a complete series of novel applications in the display and illuminating technique.
In the document U.S. Pat. No. 5,093,698 an organic pin-type light-emitting diode is described that involves an organic light-emitting diode with doped charge carrier transport layers. In particular, three organic layers are used, which are located between two electrodes. N-type and p-type doped layers improve here the charge carrier injection and the transport of holes and electrons in the corresponding doped layer. Consequently, the proposed structure consists of at least three layers with at least five materials.
The energy levels HOMO (“Highest Occupied Molecular Orbital”) and LUMO (“Lowest Unoccupied Molecular Orbital”) are preferably selected in such a way that both charge carriers are “captured” in the emission zone in order to ensure an efficient recombination of electrons and holes. The restriction of the charge carriers to the emission zone is realised by a suitable selection of the ionisation potentials and/or electron affinities for the emission layer and/or the charge carrier transport layer, as will be explained later.
The element structure as known from the document U.S. Pat. No. 5,093,698 leads to a greatly improved charge carter injection from the contacts into the organic layers. The high conductivity of the doped layers, moreover, reduces the voltage decline occurring at that location during the operation of the OLED. For this reason, doped structural elements should require significantly lower operating voltages for a desired luminance than comparable non-doped structures. Further examinations, related hereto, of such doped structural elements have shown, however, that this is not necessarily the case. In the original pin-structure, exciplex formation as well as the so-called luminescence quenching effects cannot be ruled out, and this has a negative effect on the quantum yield of the electroluminescence. Luminescence quenching occurs particularly in such a case when p- or n-dopants are in the immediate vicinity, meaning, in the organic layer adjoining the emission zone.
In the document DE 100 58 578 C2, block layers were inserted between the central emission layer and at least one charge carrier transport layer for these reasons. In this case, the charge carrier transport layers are also doped either with acceptors or donors. It is described as to how the energy levels of the block materials are to be selected in order to enrich electrons and holes in the light emitting zone. Thus, the known structure does in fact enable high efficiencies as the additional intermediate layers also act as a buffer zone to the formerly possible quenching effects at dopant disturbance locations.
A luminescence quenching can be caused by several effects. One possible mechanism is known as exciplex formation. In such a case, holes and electrons that should actually recombine with one another on an emitter molecule in the emission zone are located on two different molecules on one of the boundary surfaces to the emission layer. This so-called exciplex condition can be understood as a charge-transfer-exciton where the participating molecules are of a different natures. With an unsuitable selection of the materials for block and emission layer, respectively, this exciplex is the energetically lowest possible excitated condition, so that the energy of the actually desired exciton on an emitter molecule can be transferred into this exciplex condition. That leads to a reduction of the quantum yield of the electroluminescence and, consequently, of the OLED. In some cases the red-shifted electroluminescence of the exciplex is also observed. As a rule, however, this is then characterised by very small quantum yields.
Further mechanisms of the luminescence quenching occurring in OLEDs originate as a result of an alternating effect of excitons with charged and uncharged dopant molecules on the one hand and/or with charge carriers on the other hand. The first mechanism is effectively suppressed by means of the use of non-doped block layers based on the short range of the alternating effect. Charge carriers during the operation of the OLED must inevitably occur in and in the vicinity of the emission zone. For this reason there can only be an optimisation to that extent that an accumulation of charge carriers, for example in a band discontinuity, is avoided. This imposes, in particular, demands on the selection of the tape layers for block material and emitters in order to avoid barriers for the charge carrier injection and, subsequently, an accumulation of charge carriers.
A pin-structure according to the document DE 100 58 578 C2 already comprises at least five single layers with more than six different organic materials owing to the fact that the functionality of each single layer is closely linked to the specific energy level, as is described in greater detail in the document DE 100 58 578 C2.
A first step for simplification is provided by BPhen/BPhen:Cs layer sequences (compare He et al.: Apply. Phys. Let., 85 (17), 3911 (2004)). This system uses the same matrix material, namely BPhen, both in the electron transport layer as well as for the directly adjacent hole block layer. With this known system, however, the possible exciplex formation is not prevented because of a decisive energy level difference between LUMO of BPhen and the HOMO of the matrix used for the emission zone. In actual fact, an improvement of the structural element by means of the selection of TAZ for the hole block layer is reported. Accordingly, the layer sequence BPhen/BPhen:Cs does not correspond to the simplification of layer structures while maintaining the efficiency of structural elements, which takes place with the help of a specific selection of the materials concerned. In particular, the known system is not compatible with the combination of emitter materials selected at that location. Furthermore, the structure as described by He et al. contains at least four matrix materials.
Furthermore, a structure is known in which the emission layer and a charge carrier transport layer consist of the same organic matrix material (compare J. Kido, Proc. 1st Int. Display Manufacturing Conference IDMC 2000, Seoul, 2000). Here, a structural element emitting organic light is explained that uses an Alq3 layer as emission layer, onto which again a Li-doped Alq3 electron transport layer is adjacent. This sequence is not embedded in a pin-OLED structure, where not only acceptors in the hole transport layer are present but also donors in the electron transport layer.